1. Field of the Invention
The present invention relates to a blue light emitting material having a new crystal structure, an electroluminescent device utilizing the same and a method for manufacturing the electroluminescent device.
2. Related Art
Conventionally, an electroluminescent device is formed by sequentially laminating a first electrode, a first insulating layer, a luminescent layer, a second insulating layer and a second electrode on a glass substrate which is an insulative substrate, and at least a light outgoing side from the luminescent layer is formed by materials which are optically transparent.
The luminescent layer is formed by doping a luminescent center in a host material, and various kinds of materials have been suggested as materials for the luminescent layer. For example, Japanese Patent Application Laid-Open No. 2-72592 or Japanese Patent Application Laid-Open No. 5-65478 discloses calcium digallium tetrasulfide in which cerium has been doped (CaGa.sub.2 S.sub.4 :Ce) or strontium digallium tetrasulfide in which cerium has been doped (SrGa.sub.2 S.sub.4 :Ce), as a material for the luminescent layer.
In a case where a composition material composed of three elements such as CaGa.sub.2 S.sub.4 or SrGa.sub.2 S.sub.4 which is a host material of the luminescent layer is used as the luminescent layer of the electroluminescent device, annealing treatment is performed at a temperature of 600 to 650.degree. C. after formation of the luminescent layer in order to improve crystallinity thereof. It is to be noted that the reason why an upper temperature in the annealing treatment is 650.degree. C. is that, if the annealing treatment is performed at a temperature higher than that, a great deal of distortion occurs in the glass substrate.
Even if the annealing treatment is performed at the above-specified temperature, however, the crystallinity of the luminescent layer formed by CaGa.sub.2 S.sub.4 or SrGa.sub.2 S.sub.4 is far worse than that of a luminescent layer in which zinc sulfide (ZnS) or strontium sulfide (SrS) is used as a host material. For this reason, it is very difficult to realize an electroluminescent device having sufficient luminance for practical use by utilizing the CaGa.sub.2 S.sub.4 :Ce or SrGa.sub.2 S.sub.4 :Ce luminescent layer.